1. Field of the Invention
The invention relates to a device comprising an array of pixels which are reversibly switchable between at least a first and a second chemical state, the first and second chemical states differing in optical properties, said pixels comprising a stack of layers including a switchable layer of an optically switchable material which brings about a switch from the first state to the second state of the pixel, and a driving circuit for switching the pixels.
2. Description of the Related Art
U.S. Pat. No. 5,905,590 describes a switching device comprising a switching film including hydrides of magnesium with other trivalent metals. By exchange of hydrogen, the switching film can be reversibly switched from a first, transparent, chemical state to a second, mirror-like (fully reflecting or scattering), chemical state with zero transmission via an intermediate black absorbing state. The switching film is comprised in a stack of layers, which is deposited on a transparent substrate. By virtue of the optical effect, the device can be used as an optical switching element, for example, as a variable beam splitter, optical shutter, and for controlling the illuminance or the shape of light beams in luminaires. The switching device can also be used for data storage and in optical computing, and in applications such as architectural glass, vision control glass, sunroofs and rear-view mirrors. By making a pattern in the switching film and providing the patterned switching film with transparent electrodes, a thin display can be manufactured having an array of pixels. A thin display device conventionally comprises an array of pixels and a driving circuit for switching the pixels.
It is a problem with this type of device that throughout the device, the switching behavior of the pixels should be the same, while also a long-term stability of the pixels is required. Any difference in these properties would be or become visible to the viewer either in intensity or image differences on the display or in an ongoing degradation of the image.
It is an object of the invention to provide a switching device having a long-term stability and a good image reproduction. To this end, the invention provides a device which is characterized in that the device comprises pixels for which a voltage-limiting device is arranged parallel to the stack of layers.
Pixels are switched by supplying a switching current to the stack of layers by means of applying a voltage across the stack of layers. Application of said voltage results in a change of the chemical state of the switchable material, e.g. in the switching mirror device H-ions migrate from one H-containing layer to the other whilst an electric current flows around the device. The optical properties depend on the state of charge of this hydrogen battery or, in other words, on the chemical state of the switchable layer. Another example of switchable layers is embodied in electrochromic devices, such as disclosed, for instance, in World Patent application no. WO 98/48323 in which a stack of layers is present comprising a switchable electrochromic layer comprising a metal oxide such as tungsten oxide, molybdenum oxide, niobium oxide, manganese oxide or zirconium oxide or mixtures thereof. Applied voltages in the driving circuit will vary over the display due to current losses. Ideally, the pixels are switched fast. If a voltage which is sufficient for a particular pixel in the center of the display to switch is applied across said stack, i.e. a voltage slightly above the threshold voltage for switching a stack, other pixels closer to a primary voltage source will, due to electrical losses in the driving circuit, usually receive a much higher voltage, well above said switching threshold voltage. As the inventors have realized, applying a too high voltage across a pixel will lead to a degradation of the stack and to a loss of picture quality. The threshold voltages of the stacks are relatively close to voltage levels which will degrade the stack. If, on the other hand, the switching voltage for a pixel at the edge of the display has the right magnitude, i.e. enough to switch the stack, but not enough to degrade the stack, the switching voltages for pixels in the center of the display are considerably lower, having as a consequence that either the switches are too slow, or the stack is not switched at all. Both effects lead to errors in the image.
By introducing, according to the invention, for at least some pixels a voltage-limiting device on the stack of layers, such as preferably a diode, the voltage across the stack is limited, so that application of a too high voltage across the stack is prevented, thereby preventing degradation. This allows the use of switching voltages in the display which are anywhere in the display over the threshold for switching of the stack and over the voltages which would lead to a degradation of the stack. The voltage-limiting devices ensure that the effective voltage across the stack is kept below a voltage which would lead to degradation of the stack. Preferably, all pixels are provided with a voltage-limiting device to prevent degradation of every pixel.
It is remarked that the invention relates to devices in which, by application of a voltage, a change is produced in the chemical state of the switchable layer. Such a chemical state change is in essence a redox reaction whereby, to and for, the switchable layer transport of ions (Hydrogen, Oxygen, Lithium or others) or electrons occurs. Such devices differ within the framework of the invention fundamentally from devices in which the application of a voltage leads to a transition between two states of a material which do not differ in chemical property, but only in physical properties (for instance, birefringence), such as e.g. LCD devices. Applying a voltage in an LCD device well over the switching voltage does not normally lead (unless truly excessive voltages are applied, usually far outside the range of driving circuits) to permanent damage to the LCD layer.
A diode is preferred for use as a voltage-limiting element, since it is a simple element, which can easily be implemented. Also with present technologies, diodes are available which operate at a suitable voltage. Within the framework of the invention, a xe2x80x98diodexe2x80x99 is any element functioning as a diode, including e.g. a diode-connected transistor.
The invention is of particular advantage when use is made of an active matrix display using TFTs.
The voltage needed for switching a xe2x80x9ctransparentxe2x80x9d device is typically about 1-2 volt, and for a symmetric device it may be as low as about 300 mV.
Since the voltages needed for switching a pixel may be well below 1 V, the TFTs will have to be run with a low drain-source voltage (say, 0.3V-2 V, the maximum voltage before the pixels begin degrading) but with a high gate voltage (say, 10 V). This means that the TFTs are not operating in the current saturation range, but in the linear range, where the current can vary strongly. The current may also be lower than ideally required, which will slow down the switching. However, to obtain the desired pixel current levels (typically 50 xcexcA), a TFT typically requires a drain-source voltage of around 2-3V. The use of the voltage-limiting device on the stack in the present invention prevents the pixel voltage from increasing to this value during and/or at the end of the charging cycle, which would otherwise happen, and thereby prevents the display device from degrading strongly or being destroyed while yet enabling an active matrix TFT circuit to be used, which is very advantageous for this type of display device.
Preferably, two current-limiting devices are arranged in parallel on the stack to limit the voltage across the stack during charging and discharging.
In a preferred embodiment of the invention, one side of the voltage-limiting devices is connected to a reference voltage point or line in the driving circuit. This reduces the number of connections to the stacks and makes it possible to adjust the reference voltages and thereby adjust the limiting voltage across the stack by adjusting the reference voltage.